There was a massive wave of phones being sold with unlocked bootloaders with preinstalled malware. How do you stop this? Unscrupulous retailers would buy thousands of your latest phone, root it and sell it for $1 less than MSRP, and make much more than that from injecting extra ads and stealing data. Not to mention, all the bad press "you" get for "shipping a phone with ads in it".
Mere tamper-evidence - having a "bootloader unlocked" warning on boot - isn't obvious enough for a new user who has just bought the phone for the first time. So all manufacturers have added time locks and anti-fraud detection for bootloader unlocking. The time lock in particular is the best way to cut down this effect during the early sales process after which time it's hard to catch up in review score. But a time lock can only be implemented securely via a remote server out of the user's hands.
Xiaomi getting the telemetry is a cherry on top, but it's not originally nefarious - or at least - it's less nefarious than the problem it's intending to solve